I think all this would have been due to a virus.
Though any antibiotics can cause loose stools, Zifi- cefixime
usually does not.
The pain in his tummy might be due to the infection of the intestine he has. Since you have started the antibiotic, complete the full course.
See your doctor again for his pain in his tummy. He might need medications like Cyclopam
in recommended dosage to relieve the pain.
You may give him probiotic
sachets like econorm/ bifilac
Along with the medications give him plenty of curd and other fluids like soups. rice based porridge, banana etc. Curd has probiotics naturally and will be of help in loose stools.
Hope you find my answer helpful.
Wish a speedy recovery for your kid.